Lifting rail alternative

Discussion in 'Powerboats' started by Toolate, Sep 30, 2013.

  1. Toolate
    Joined: Sep 2013
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    Toolate Junior Member

    I just bought a 30 Sisu which is a round chine downeaster with full keel and WOT speed of 14kts but I plan to replace the engine and get her to druise around 16 with WOT over 20. The new higher cruise might (dont know but it seems to be recommended) necessitate lifting rails to get the boat up a bit inthe rear and level her out. Mostly all I am interested in is efficiency- would love to save a couple gph if possible.

    So, the conventional lifting rail is a pvc extrusion that many people bolt on or glue or just make in wood and glass on. Aft section is usually below the water line by a couple inches but still on the the side of the chine- not around the bottom and this requires that they be blocked when the boat is lifted in slings and it also makes a visible statement that I dont really like. Plus there is no way I am bolting below the waterline...

    I have been thinking that I could mold some strakes for the last half of the wetted area on my boat. Maybe make them in wood or even in hull and deck putty and then glass them over. All below the water line so I could leave my topsides undisturbed.

    Dimensions would be about 3/4" steps maybe 3" apart and stepping up like clapboard on a house/strakes on a lapstrake hull. 3 x 3/4" would add up to more than the 2' or so that most lifting rails measure.

    Any thoughts? Anyone ever do this or even glass over a lapstrake hull and lose the strakes and have some thoughts on the loss of performance?

    The old wedge trick would be great but there is no way to test it and trim tabs arent so down-easty. Thoughts?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Below waterline strakes won't help you, imo, they serve chiefly to reduce wetted area on fast boats, which does not apply here. Lot of work for no gain, imo.
     
  3. DCockey
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    DCockey Senior Member

    Eric Sponberg's website has a page about a boat he designed modifications for which significantly improved the speed of the boat. My recollection is the boat had round chines was modified to have sharp chines. Unfortunately I can't currently access his website.
     
  4. DCockey
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    DCockey Senior Member

  5. PAR
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    PAR Yacht Designer/Builder

    Post a photo of the boat. It's probable that you have a semi displacement and too much rocker to push the boat much faster then it is now, even with strakes.

    [​IMG]

    If it's the hull form I suspect, you're not going to see 20 knots, without an atomic reactor. It's simple physics and flow dynamics. As you can see, this Sisu is at WOT (220 HP diesel, running at about 16 knots) and can't carry her bow, because she'd trim too high to do it. If the aft portions of the bottom where a different shape (straightened buttocks) you could, but without significant changes to the hull form aft, you're married to her hull form limitations.
     
  6. boatbuilder41
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    boatbuilder41 Senior Member

    I made some multi purpose rails on my last build and it was a amazing difference. I have not seen any exactly like the ones I made, and I wasn't sure how it would work. I knew that if I created lift, I could gain speed [in most cases] . I had a lot of time to study how I wanted to do this. It t.ook me 14 months from start to finish, mainly because I w.ork alone, from start to finished product. I had to modify a little after my first test run. And forgive me ....I know this will not work in all applications. And I know some will disagree with me, but I increased speed by 6 knots, better stability at planing and non planing speeds, and spray. I know the boat you are talking about has round chines and that makes for a whole new ball game. But maybe I can help you create an idea. I placed mine right on the chine at a 7 degree angle [totally experimental] they went all the way to the stern[mistake].I started at 6 inches wide [mistake]. I gained so much positive lift that the rudder wouldn't even hold the boat straight enough to navigate on a plane. Totally dangerous until I cut off the last six foot. . It worked out perfect for my use. I forgot to mention that I made them of solid fiberglass and glassed them to the boat
     

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  7. Barry
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    Barry Senior Member

    An excerpt from another article.
    "The problem is that the hull, with it's round chines and prop pockets doesn't have enough surface to plane effectively, and instead just digs itself into a hole as you apply more power beyond it's hull speed.

    That's why Bayliners (older round chined Bayliners) have such a reputation as causing monstrous wakes. It's not entirely the owner's fault.

    The solution? More lift aft. Sounds simple. And it is, kinda. John Ripley ay Banana Belt Boats used to work for one of the largest Bayliner dealers around, and he developed a hull addition that creates a hard chine aft, more lift and a better running boat.

    North Harbor Diesel in Anacortes has installed a lot of these. Howard at North harbor, or john at Banana Belt can give you performance improvement numbers. With the bigger Hinos and the gas small blocks it can be a sizable increase requiring props be repitched because the boat moves through the water more easily."

    You can check these out. The information pertains to older Bayliners which were built with round chines, maybe mid 80's from their 32 foot up to the 49.

    Re lift strakes, they work well under water.

    People often use speed and velocity as if they are the same parameter. They are not.
    Speed is just say miles per hour, or feet per second.
    Velocity is miles per hour, north, or feet per second south.

    Force equals mass times acceleration. F=ma

    If a mass is moving (the water) and you accelerate it, the result will be a force

    Acceleration is a change in VELOCITY
    Velocity has two components, speed and direction. Therefore if you change the direction, then you have accelerated the mass, and hence get a resultant force

    If a hull is planing, the water is moving from the keel toward the chines at an angle, dependent on boat speed, hull design. So if you install strakes, this water that is moving outboard will hit the strake, change direction and give lift.

    On planing hulls that we built, with a 12 degree deadrise and a 7 degree down angle on the strake

    The Bayliner hard chine modification, reduced the coanda effect which robs lift, and increased the width of the hull at the chine. All of these changes are positive.
     
  8. CEShawn
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    CEShawn New Member

    IMO on a soft chine boat, they have to make a difference. I'm still learning though and this website has answered a lot of my questions. The difference though will be how notable when you are only dealing with lets say 17 knots? Again I'm just trying to make sure it beats out a cost average of the job in a year time span. Take a 1000hrs a year, at $4gallon, any savings on that isnt bad. The sad math to that is only about a 1/3 of the time is spent at a higher speed where at 5kts not much gain is going to be noticed.

    More importantly, let me ask you this... How about placement? Sure the camera idea is great but even at that, I believe you are trying to break the surface tension (granted my term might be off)

    Usually I have seen on DE boats the lifting rail end, I think I just learned guidance of ending it where the keel ends. My last boat HH40 ended at the stern, picture enclosed.

    Now where do I put mine now? ? ? Hang upside down with a scribe while doing my cruising speed?
     

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  9. boatbuilder41
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    boatbuilder41 Senior Member

    Its just going to be trial and error if you are going to get maximum performance. I know my first test run was awfully dangerous. My theory as make it big and cut it off. Just to prevent from having to add more or do any more laminating. The rudder is 22 inches high and 18 inches long, and it wouldn't even hold the stern from passing the bow. It was crazy. I would have never believed the boat would try to swap ends with me. And if it got sideways at 30 knots..........it would have been the end. The rail being inverted and 6 inches wide, and glassed on right at the chine, would have no choice but to dip the side, and that would have been a bad mistake. I wouldn't suggest going to extremes like I did. Nothing would have ever made me believe that you could gain so much lift that it was impossible to navigate at around 75% and up.
     
  10. boatbuilder41
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    boatbuilder41 Senior Member

    I actually have video of how it performed and how much difference it made, If anybody would like to see them just message me. I haven't figured out how to post a video on here
     
  11. Toolate
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    Toolate Junior Member

    SO its been a while but I am finally done with a HUGE project repowering and refitting this boat and the same question persists. I think the strake idea would be interesting to try but I am going to take all the advice I have heard which is that it wont.

    New power is a mercruiser 383 (375 hp) MPI with ZF 630a 2.54:1 gear and 17x20 4 blade prop that makes about 21.5 kts (!) at WOT of 5200 rpm. Same hull PAR as you pictured but I suspect mine is heavier with the bridge and interior. That boat is a bare bones fishing boat (I have seen pics on another site).

    So, couple questions-

    Unlike the boat pictured above, mine runs bow high- maybe 6 or 7 degrees more (see pic). I suspect that lifting rails would do this boat a great service by offering some lift aft and giving it some stability at speed as it currently rolls/wallows a bit although I suspect that some of this is because I am pushing it past hull speed. At 21 kts its still steady though with no feeling of wanting to keel walk.

    I would love to avoid adding lifting rails but think its the only way to get the boat to do a little better. Any thoughts on the placement/size or reference info I could read?

    I would love to form these in foam and then glass them over- hull is cored and I will not bolt through it since theres no access to the inside. Would love some suggestions on what kind of foam and layup schedule. I want something I can not worry about in the slings (blocking etc.. if possible)

    Boat is fully built down hull form so I understand that I am past the hull/design speed on all of this but the fact is that it runs relatively efficiently (12gph at 14kts) and I think some rails/added lift would improve this even more by getting the stern up a bit. Almost feels like the prop is sucking the back down by creating a negative pressure under the boat and I think the rails might raise this a bit.

    Any and all suggestions are welcome from "thats a stupid idea" to "do this" :).

    Hey CEShawn! Same Toolate....
     

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  12. PAR
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    PAR Yacht Designer/Builder

    A hook would be a more appropriate and effective addition to the hull. Getting lifting strakes shaped right, to do what you want is part science and part black magic. Designing a hood, may be just as simple as gluing on some wedges aft, at the transom, playing with thicknesses and tapers, until you get what you want. Once the temporary wedges are finalized, you can 'glass in some foam and fabric to lock down your choices. The wedges will shave a bit of top speed (maybe a MPH or two), but she'll bring her bow down and become more dynamically stable.

    The photo shows the boat dangerously down at the stern, nearly getting swamped. With her running more level (3-4 degrees bow up), you'll clear the transom a lot and she'll ride much better too. Lastly, what's the real difference between 19.5 MPH and 21 MPH, especially if the boat is trimmed right, more comfortable and handles better.
     
  13. Toolate
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    Toolate Junior Member

    SHould have mentioned that I put the volvo tabs on the boat and they do make a difference in how the boat feels but dont change the attitude enough in my opinion. I think the pic makes it look lower than it is but it is low.

    PAR you would opt for a wedge over a lifting rail? Seems to me I need all the help I can get and to my inexperienced brain a rail would provide more lift over a larger area than a wedge. PLus I have tried the Volvo Tabs and they create what I imagine are two smaller areas of increased pressure under the hull and I wonder if the rails might be the next step.

    I am all ears. Thanks for your time and enerygy.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Again, designing lifting strakes is quite difficult to do right and a novice would simply be guessing, which generally is an expensive way of doing things.

    How much deflection are you putting into the tabs currently? What size are the tabs?
     

  15. DCockey
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    DCockey Senior Member

    The Sisu 22 originally had soft, rounded chines like the Sisu 30. The current version of the Sisu 22, now built by Eastern, has lifting strakes almost the entire length of the boat which can be seen in this photo from the Eastern website: http://easternboats.com/wp-content/gallery/22-sisu/DSC_0442.jpg At the Annapolis Powerboat Show I talked with the Eastern rep who had worked for the original builder of the Sisu 22 three decades ago. He said the addition of the lifting strakes to Sisu 22 hull which was done by the one of the early builders resulted in a major improvement to the boat's overall performance.
     
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